1. Field of the Invention
The present invention relates to electrical heating cables that use positive temperature coefficient thermistors as self-regulator heaters.
2. Description of the Prior Art
As exemplified in U.S. Pat. No. 4,072,848, electrical heating cables have been used commercially for some time to provide heat to pipes and tanks in cold environments.
Heating cables as disclosed in U.S. Pat. No. 4,072,848 based their temperature control on the use of variable resistance heating materials which provide a self-regulating feature. The heating materials are generally formed into chips made of barium titanate or solid solutions of barium and strontium titanate which are made semiconductive by the inclusion of various dopants. These chips are referred to as positive temperature coefficient thermistors and have a relatively low temperature coefficient of resistance at low temperatures. As the temperature of the thermistor rises, a sharp rise in the resistance occurs at a point termed the "Curie point". The transition from low resistivity to high resistivity occurs at a relatively sharp point as shown in U.S. Pat. No. 4,072,848. As these chips are well known to those skilled in the art, no further discussion of their construction is necessary.
As a voltage is applied to the thermistor, the thermistor generates heat due to resistance effects. This heat is then transferred to the environment and used to heat up the surrounding environment, such as the pipe to which the cable is attached. As the temperature of the thermistor and the surrounding environment increases, the thermistor temperature reaches the Curie point, the heat producing capability of the thermistor is reduced and the thermistor cools down. Thus the thermistor temperature settles on or near the Curie point, with the temperature of the surrounding environment being based on the thermal conductivities of the various materials in contact with the thermistor.
Prior art thermistor-based heating cables had the problem of relatively low overall efficiencies because of the limited heat transfer from the thermistors to the surrounding environment. This limited heat transfer occurred because the thermal conductivity from the thermistor to the environment was relatively low, causing the thermistor temperature to rise to the Curie point or switch temperature at a lower total power output than would occur if good heat dissipation existed.
Additionally, conventional designs have not had a uniform temperature distribution without the need for a large number of thermistors, in part because of the poor thermal transfer properties of the materials used in constructing the cables.
U.S. Pat. Nos. 4,117,312, 4,250,400 and 4,304,044 attempted to solve the temperature distribution problem by the use of resistance wire connected between a thermistor chip and the various conductors carrying the voltage from the power source. In this way, the resistance wire performed the bulk of the heating and the thermistors were used as switches to switch in and out resistance wire legs. Non-resistance wire thermistor-based heating cables tended to have hot spots near the thermistor because of poor heat distribution throughout the length of the cable, so that hot spots developed and non-uniform heating of the environment occurred. The use of the resistance wire provided a more even distribution of produced heat, but had the disadvantage of requiring additional wire and components to produce a heating cable.
U.S. Pat. No. 4,104,509 attempted to resolve the heat transfer problem by using heat conducting, electrically insulating compounds of silicone rubber, magnesium oxide and silicone oxide or other compounds in the heating element casing to provide better heat dissipation for the thermistors. The use of this design required the use of additional materials from the simple design as shown in U.S. Pat. No. 4,072,848. Additionally, the suggested materials were hygroscopic, requiring water tight sealing of the heating element casing to allow proper, continued operation.
British Patent No. 1,306,907 used a rigid casing with an electrically insulated liquid to improve the heat transfer from the thermistors to the environment. This design had the problems of requiring additional components and the casing was rigid for proper operation, therefore limiting the uses of the cable to non-flexible applications.
U.S. Pat. No. 4,072,848 indicated that the conductors assisted the thermistors in heat dissipation. The conductors disclosed in U.S. Pat. No. 4,072,848 had a small surface area and small contact area with the thermistor so that the heat dissipated and transferred along the conductors was relatively limited. The dielectric or insulation materials were the primary means of heat conduction and the poor heating pattern and low thermal conductivity developed because of the poor heat transfer properties of the dielectric materials.
Additionally, the previous designs using thermistors in flexible heating cables induced large thermal and mechanical stresses on the mating surfaces of the thermistors and the voltage source conductors. This limited the flexibility or sizing of the copponents in the heating cable.